This invention relates in general to a seal and more specifically to a non-contacting device that prevents a liquid such as oil from escaping from an enclosed cavity such as a gearbox along a rotatable shaft.
Gearboxes typically use oil as a lubricant both between the gear teeth and in the bearings that support the shaft. The level of the oil is below the opening for the shaft, but typically when operating the gearbox is filled with a mist of oil. Oil is also splashed throughout the gearbox either by the action of the gear dipping into the oil or by a pressurized lubrication system that supplies oil to the bearings and sprays oil onto the gears. Leakage of this oil can cause environmental and economic problems. For example, in many high-speed centrifugal compressors the impeller is mounted directly on the output shaft of the gearbox. When the compressor is unloaded any oil that gets out of the gearbox may get sucked into the compressor and contaminate what should be an oil free area. When the compressor is subsequently loaded, the oil can travel with the compressed gas and contaminate the process. This is unacceptable in many plants, such as in pharmaceutical manufacturing.
One type of seal used extensively to prevent oil leakage from a gearbox is the mechanical seal. This type of seal, however, is speed limited and therefore cannot be used for very high speed shafts such as those used in centrifugal compressors. This is because typical mechanical seals generate excessive heat due to the mechanical shearing of the oil at high peripheral speeds. Mechanical seals may also wear over time, and need periodic replacement.
Another seal type that is well known in the art is the labyrinth seal. Since this type of seal is non-contacting, it is used extensively on high-speed equipment. Unfortunately, they are not very good oil seals unless either a buffer gas is used to assure that there is a continuous flow of gas towards the gearbox to prevent the oil from migrating out or enough axial space is provided for at least two sets of teeth with an oil drain between them. Use of a buffer gas adds to both initial, ongoing and maintenance costs, while lowering reliability. Adding additional axial space may cause rotodynamic problems, as well as added cost to the system.
As can be seen from the above description, oil seals in the past have worked with varying degrees of efficiency, but have either required external support, lacked reliability or required a large axial space.
It is therefore an object of the present invention to provide a seal means which overcomes the problems of the prior art described above.
It is another object of the present invention to provide a seal which prevents oil leakage from an enclosure along a rotatable shaft.
It is a further object of the invention to provide a simple seal design that does not rely on external support for its proper operation.
Another object of the present invention is to provide a seal design which minimizes the axial length needed for the oil seal.
These and other objects of the present invention are obtained by the configuration of a rotatable shaft operating in conjunction with stationary seal teeth connected to a housing or suitable support.
The present invention is directed to an annular non-contacting seal comprising a stationary tooth that radially overlaps a tooth on a rotatable shaft, with the sealing action resulting from controlled clearance both radially between the stationary tooth and the shaft and axially between the stationary tooth and the shaft. In a second embodiment the non-contacting seal may comprise a stationary tooth that is approximately in the same radial plane as a tooth on a rotatable shaft, with the sealing action resulting from both the controlled clearance between the two teeth and from the momentum of the liquid traveling outward from the rotating tooth making it difficult for the liquid to turn to go axially through the gap between the rotating tooth and the stationary tooth. In a preferred embodiment, the seal may comprise the combination of the seals in the two embodiments described above. The stationary teeth which comprise the seal may be made of any suitable material. Typical materials include metals, plastics and ceramics. In one embodiment the seals were made of an aluminum alloy.
The seal functions to prevent oil leakage from an enclosure adjacent or surrounding a rotatable shaft such as that used in a high speed centrifugal compressor in which an impeller is mounted on the output shaft of a gearbox.